3 Answers
3

Unix was and is (mostly) proprietary. So some people got their collective noggin together in the 1980s and put together something that reproduced Unix functionality without copying its source code, which would have been illegal [insert longer version about Richard Stallman and Linus Torvalds here].

One of the first GNU/Linux distributions was Debian. Ubuntu was created by people that had been involved with Debian and Ubuntu is officially proud of its Debian roots. It's all ultimately GNU/Linux but Ubuntu is a flavor. In the same way that you can have different dialects of English. The source is open so anyone can create their own version of it.

However, Debian is not really useable for mere mortals such as myself and that is where something like Ubuntu comes in. Indeed, I understand that it is specifically the strategy of Canonical, the company behind Ubuntu, to target the non-geek user. Which is why most new GNU/Linux users come to Ubuntu or Mint (which is based on Ubuntu) first.

I would add that Ubuntu and Debian are, namely, distros, which means that they use Linux and a collection of core Linux software to provide a GUI/server software with a terminal, shell, etc... Linux on its own won't do much to serve HTTP requests or even manage packages like apt.
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hexafractionSep 3 '12 at 20:42

The Unix operating system was conceived and implemented in 1969 at
AT&T's Bell Laboratories in the United States by Ken Thompson, Dennis
Ritchie, Douglas McIlroy, and Joe Ossanna. It was first released in
1971 and was initially entirely written in assembly language, a common
practice at the time. Later, in a key pioneering approach in 1973,
Unix was re-written in the programming language C by Dennis Ritchie
(with exceptions to the kernel and I/O). The availability of an
operating system written in a high-level language allowed easier
portability to different computer platforms. With a legal glitch
forcing AT&T to license the operating system's source code to anyone
who asked,[22] Unix quickly grew and became widely adopted by academic
institutions and businesses. In 1984, AT&T divested itself of Bell
Labs. Free of the legal glitch requiring free licensing, Bell Labs
began selling Unix as a proprietary product.

Linux is a Unix-like computer operating system assembled under the
model of free and open source software development and distribution.
The defining component of Linux is the Linux kernel, an operating
system kernel first released 5 October 1991 by Linus Torvalds. Linux
was originally developed as a free operating system for Intel
x86-based personal computers. It has since been ported to more
computer hardware platforms than any other operating system. It is a
leading operating system on servers and other big iron systems such as
mainframe computers and supercomputers:more than 90% of today's 500
fastest supercomputers run some variant of Linux,including the 10
fastest. Linux also runs on embedded systems (devices where the
operating system is typically built into the firmware and highly
tailored to the system) such as mobile phones, tablet computers,
network routers, televisions and video game consoles; the Android
system in wide use on mobile devices is built on the Linux kernel.

Ubuntu is a computer operating system based on the Debian Linux
distribution and distributed as free and open source software, using
its own desktop environment. It is named after the Southern African
philosophy of ubuntu ("humanity towards others"). As of 2012,
according to online surveys, Ubuntu is the most popular Linux
distribution on desktop/laptop personal computers, and most Ubuntu
coverage focuses on its use in that market.

Ubuntu is sponsored by the UK-based company Canonical Ltd., owned by
South African entrepreneur Mark Shuttleworth. Canonical generates
revenue by selling technical support and services related to Ubuntu,
while the operating system itself is entirely free of charge.